In hydraulic pumps there is an ever present objective for improving volumetric and overall efficiency. This objective has been accentuated in recent times by the need to consume energy. In the past, it has not been practical to obtain overall efficiencies of more than 75% for high pressure vane type pumps but now with the need to consume energy higher efficiencies are greatly desired.
In hydraulic pumps, the volumetric efficiency is the percentage of fluid by volume that is discharged through the outlet port as compared with the volume coming into the suction side of the pumping chamber. Some of the fluid leaks from the pump pressure cavity through various leakage paths between the pump parts and does not reach the outlet port. This leakage fluid is referred to as hydraulic slip and the leakage fluid is recovered either by re-pumping it back to the pressure cavity or by draining it from the pump housing to a reservoir. The greater the slip the less the volumetric efficiency and consequently a lower overall efficiency for the pump.
In vane pumps a large part of the slip is through the necessary clearances that must be established between the side surfaces of the rotor and the adjacent flat faces of the side plates. In the case of variable vane pumps, there must also be clearances between the sides of the cam ring and the adjacent flat surfaces of the housing or side plates. It is necessary to have clearances at the locations referred to in order that the rotor may rotate freely without binding and in order that the cam ring may be moved freely without sticking or binding for varying the volume of the pump. A practical clearance in these locations for avoiding binding or sticking of the rotor and cam ring may be, for example, about 0.0004 inch in a pump for delivering 65 gallons per minute at 2,500 psi.
Such a clearance of about 0.0004 inch has been not only difficult to obtain initially because of manufacturing variations in the parts, but also as been materially affected by temperature differentials created within the pump during operation. Thus, for example, differential expansion of the rotor relative to the spacer due to temperature conditions may be as much as 0.0008 inch or more. To accommodate this differential expansion the total clearance on the two sides of the rotor must be increased by 0.0008 inch so that when the parts change dimension in a manner to decrease the clearances because of thermal conditions during operation, the initial clearance on each side must be 0.0008 inch instead of the desired 0.0004 inch. The importance of maintaining a very small clearance during operation of the pump is accented when it is realized that at a pump operation pressure of 2,500 psi the leakage or slip is more than 8 times greater through a 0.0008 inch clearance than through a 0.0004 inch clearance. If the rotor does not remain centered so that all of the additional 0.0008 inch clearance occurs at one side, the slip becomes 14 times greater.